if strcmp(signal, 'off') 
    I = std_IQ*randn(M, N) ; 
    Q = std_IQ*randn(M, N) ; 
    X2 = I.^2 + Q.^2; 
    X2max(t) = max(max(X2)); 
elseif strcmp(signal, 'on')
    dtau = tau(t) - tau_tilda_m; 
    domega = omega(t) - omega_tilda_m; 
    I = A*L/2 * ro(dtau) .* sinc(domega*T/2 /pi) .* cos(domega*T/2 ...
        + dphi(t))  + std_IQ*randn(M, N) ; 
    Q =- A*L/2 * ro(dtau) .* sinc(domega*T/2 /pi) .* sin(domega*T/2 ...
        + dphi(t))  + std_IQ*randn(M, N) ; 
    X2 = I.^2 + Q.^2; 
    X2max(t) = max(max(X2)); 
end 